<!DOCTYPE html>
<html>

<head>
  <meta charset="UTF-8">
  <title>03_回到顶部</title>
  <style>
    #to_top {
      width: 30px;
      height: 40px;
      font: 14px/20px arial;
      text-align: center;
      background: #06c;
      position: fixed;
      cursor: pointer;
      color: #fff;
      left: 1250px;
      top: 500px;
    }
  </style>
</head>
<body style="height: 2000px;">

<div id="to_top">返回顶部</div>

<script src="jquery-1.10.1.js"></script>
<script>
  $(function () {
    //回到顶部
    $('#to_top').click(function () {
      var $body = $(document.body)
      var $html = $(document.documentElement)

      //使用scrollTop(): 瞬间滚动到顶部
      // $('html,body').scrollTop(0)

      //使用scrollTop(): 平滑滚动到顶部
      var offset = $body.scrollTop() + $html.scrollTop()
      if(offset===0) {
        return
      }
      var totalTime = 300
      var intervalTime = 30
      var itemOffset = offset/(totalTime/intervalTime)
      var intervalId = setInterval(function () {
        offset -= itemOffset
        if(offset<=0) {
          offset = 0
          clearInterval(intervalId)
        }
        $('html,body').scrollTop(offset)
      }, intervalTime)

      //使用动画: 平滑滚动到顶部
      // $('body,html').animate({scrollTop:0},300)
    })
  });
</script>
</body>

</html>